This project is concerned with a comparative analysis of ionic current channels in nerve and heart cell membranes and the relationship of these channels to electrical excitability in both preparations. During the past year the primary experimental preparations have been squid giant axons and embryonic chick heart cells. One major finding has been the discovery of Wenkebach firing patterns in the squid axon in response to periodic current pulse stimulation. This phenomenon, which has not previously been characterized in this preparation, may be relevant to cardiac arrhythmias. In the chick heart cell preparation, we have discovered a developmental loss of a particular type of calcium ion channel, the IT component, during the first week of embryonic development from atrial and ventricular heart cells. This project also has a major emphasis on computer simulations of electrical activity in nerve and heart cells. A major advance in this part of the project is the development of a model of the action potential in guinea-pig ventricular myocytes based on experimental measurements of underlying ion current components in the literature.